If Citra is already running, restart it first.
Navigate to Emulation -> Configure and in the Debug tab, click on Open Log Location.
Run the game you are having issues with until it reaches the point where it crashes or encounters the bug. Exit out of CItra.
In the window that you opened in Step 1, the log will be in citra_log.txt. Do not relaunch Citra or else the log file will be overwritten. Upload (drag & drop) this file to your support post as a file attachment.